WADA director-general David Howman says Armstrong's interview with Winfrey is "hardly the same as giving evidence to a relevant authority" that deals with doping rules and sanctions.
"He's got to follow a certain course," Howman told The Associated Press.
"That is not talking to a talk show host."
Armstrong was stripped of his seven Tour de France titles and banned for life from Olympic sports following a US Anti-Doping Agency report that portrayed him as a serial drug cheat.
After years of denials, he confessed to doping during an interview with talk show queen Oprah Winfrey that is set to air on Friday (AEDT).
Armstrong reportedly is seeking to mitigate his life ban so he can return to competing in triathlons.
Howman said a reduced ban is possible depending on the level of co-operation.
"Is he trying to do something for himself to have the sanctions changed?" Howman said.
"Does he want to do something for the benefit of the sport itself? In both instances, he will need to make a full statement on oath."
The International Cycling Union, meanwhile, urged Armstrong to testify before its independent commission on doping to shed light on allegations that include whether the UCI helped cover up his use of performance-enhancing drugs.
The UCI has been accused of covering up suspicious samples from Armstrong, accepting financial donations from him and helping him avoid detection in doping tests.
Armstrong is reportedly considering testifying against UCI officials.
Former UCI president Hein Verbruggen said he wasn't ready to speak about the Armstrong case.
"I haven't seen the interview. It's all guessing," Verbruggen told the AP.
"After that, we have an independent commission which I am very confident will find out the truth of these things."
The three-member UCI commission, chaired by retired British judge Philip Otton, will meet in London from April 9-26, with a June 1 deadline to deliver its report.
International Olympic Committee vice-president Thomas Bach said Armstrong should provide a complete confession to USADA or WADA.
Bach, a German lawyer who leads the IOC's anti-doping investigations, said a limited admission of doping by Armstrong will not be enough.
"This is not new," he said.
"If he says in general terms that he used prohibited substances, for himself and his reputation it would come a little late.
"For the fight against doping, it would not help.
"He has to show how he managed to get around the tests and whether there was somebody who helped him.
"If he is ready to contribute and say, 'I want to help my sport become cleaner,' this for me would be very much different."